Trying to help out a buddy with his LT155. Engine runs perfectly but it drives very slowly in both forward and reverse. Checked linkage and it looks good. Should I just replace the drive belt and hope for the best? Is there a way to test/observe if the drive belt is slipping?

Is it likely an issue in the hydro unit?

The machine was owned by his father since new and gently driven and well maintained but I suspect it t could be the original drive belt.